The present invention relates generally to the field of high speed optical communication and more particularly to high speed optical transmission systems utilizing dense wave division multiplexing (DWDM) and other like transmission techniques.
In optical communications systems or optical networks utilizing dense wave division multiplexing (DWDM) and other optical transmission techniques, a laser diode is used to convert electrical signals to optical pulses for transmission over fiber optic cable. To a certain extent, most laser diodes have one or more of the following drawbacks when used for optical transmission, even without data modulation being applied: 1) Random optical amplitude fluctuations referred to as relative intensity noise (RIN) amplitude; 2) Random optical phase fluctuations, where the optical phase noise is also related to optical frequency noise; and 3) Random polarization fluctuations which result because of random polarization phase changes or individual random amplitude changes of polarization states or both.
In systems using optical amplifiers between transmitter and receiver, the gain from an amplifier is dependent on the state of polarization of the light entering the optical amplifier. If the optical amplifier polarization state matches with that of the polarization state of the incoming light, a maximum possible gain may be seen at the output of the light which is desirable. Prior art methods of controlling the polarization state of light entering optical amplifiers include the utilization of polarization maintaining fiber. This type of fiber is, however, significantly more costly than traditional fiber optical cable, and its usage is therefore impractical for many applications.
Data transmission in long fiber transmission paths (such as undersea or continental terrestrial cable or lightwave transmission paths) are subject to signal fading and accompanying fluctuations in the signal-to-noise ratio (SNR) that are caused by effects of polarization. In a long lightwave transmission system with optical amplifiers, the SNR can fluctuate in a random manner, which causes signal fading. Signal fading causes delays in the data transmission channel, particularly in channels with long fiber-optic transmission paths. When the SNR of a signal in such a lightwave transmission system becomes unacceptably small, a signal fade has occurred.
Signal fading and the underlying SNR fluctuations are caused by a number of polarization dependent effects induced by the optical fiber itself and other optical components (e.g., repeaters, amplifiers, etc.) along the long optical fiber transmission path. In systems using optical amplifiers between the transmitter and the receiver, the gain from an amplifier is dependent on the state of polarization (SOP) of the lightwave entering the optical amplifier. Optical amplifiers reduce the effects of signal fading and rectify the delay problem due to long fiber-optic transmission paths. For optimal signal performance, the SOP of the optical amplifier matches with that of the incoming lightwave so that a maximum possible gain is achieved at the output of the lightwave. The SOP of the lightwave is determined by the shape of the ellipse, i.e., the direction of the major axis and the ratio of the major axis to the minor axis Eoy/Eoy, and the phase difference
PhasePolarization=phasexxe2x88x92phasey
Random polarization fluctuations result because of the random polarization phase changes or individual random amplitude change of polarization states, or both. In particular, signal fading due to polarization dependent effects over long optical fiber transmission paths can be attributed to polarization dependent loss (PDL), polarization dependent gain (PDG), polarization mode dispersion (PMD) and polarization dependent hole-burning (PDHB). All of these effects impact signal transmission as a function of the particular SOP of an optical signal being propagated along the long optical fiber transmission path.
A conventional solution to rectify the channel delay problem due to SNR fading is to simultaneously launch two signals of different wavelengths and substantially orthogonal relative polarizations into the same transmission path. Since the two signals are launched with equal power and orthogonal SOPs, the overall transmitted signal is essentially unpolarized. This has the advantage of reducing the deleterious effects of the transmission fiber""s non-linear signal-to-noise interactions, and signal delay caused by PDHB. Even though the average SNR performance improvement with such a transmitter can be substantial, such a system is still subject to substantial signal fading and in-channel delay. However, the two-wavelength source is still subject to SNR fading. Moreover, it is costly and a waste of power by using two wavelengths to transmit data because only half of the wavelengths carry useful information. Hardware resources such as dispersion compensation fiber, manual compensation tracking or variable dispersion compensation per channel in WDM are expensive and burdensome to implement. In particular, two-wavelength dispersion compensation technique for reduction of signal fading becomes prohibitive because information translation from one channel to another channel on a real-time basis is extremely difficult to perform due to channel ranges and non-linearities in WDM.
In addition to the above limitations which are present with the laser diode when used in connection with high speed optical communications, the fiber optic cable as used in the transmission introduces certain impairments into the data due to the non-linearities in the fiberoptic cable itself. As would be understood, the impairments may include dispersion, self phase modulation (SPM), cross phase modulation (XPM), FWM, etc.
The high speed optical communications which takes place within the optical systems and networks utilizing the laser diodes and fiberoptic cable may be transmitted using any one of a number of transmission coding techniques. Unipolar return-to-zero (URZ) and unipolar non-return-to-zero (UNRZ) techniques are commonly used in optical data transmission because of their unipolar characteristics. Since laser power is either zero or a certain positive quantity, only unipolar encoding can be implemented in fiber-optical communication systems. UNRZ is a widely used optical communication technique for laser modulation in optical communications because of its low bandwidth requirement as compared to URZ. URZ offers some advantages when used in fiberoptic systems with optical amplifiers; however, this use is at the cost of higher bandwidth.
High speed optical transmitters in the prior art have adopted one or the other coding techniques depending on a particular application and the amount of resources available, e.g., power, bandwidth, etc. The above discussed drawbacks and limitations associated with laser diodes and the corresponding optical networks are present, however, regardless of which of the coding techniques is utilized.
Accordingly, there is a need in the art for an optical transmitting device which combines advantages with respect to transmission of both the URZ and UNRZ coding techniques, while at the same time eliminating PMD, PDL, PDG, PDHB, XPM, SPM, FWM, etc.
The present invention is a transmitter device and related system and method for use in high speed optical transmission systems. In one exemplary embodiment of the present invention, light from an optical source, for example, a laser diode output is split into two branches of differing polarization type of substantially equal power, for example, orthogonal polarization (Eox and Eoy). Polarization selectors coupled thereto are operable to select a specific polarization to be output to a corresponding external optical polarization modulator. A first polarization modulator, receives light from the polarization selector, for example, Eox, and modulates a first coded data type, for example, URZ data from a URZ coder thereon. A second polarization modulator receives light from the second polarization selector, for example, Eoy, and modulates delayed first coded data, for example, delayed URZ data (URZd) representative of the URZ data from the URZ coder thereon. The URZ data and the delayed URZd data may be combined at the transmitter, fiber channel or receiver for transmission over a single optical medium, for example, a fiber optic cable. The net delay at the receiver in the delayed URZd data is preferably T/2, where T is a pulse period of URZ data. The first and second sets of data originating from the first and second branches of the transmitter, respectively, are uniquely added together at the transmitter, fiber or receiver in order to realize a second coded data type, for example, UNRZ data. With polarization dispersive optical fiber cable between the transmitter and receiver the Eox and Eoy polarization signals encounter different propagation delays and arrive at the receiver separately. By changing the optical polarization delay in one or the other or both arms of the receiver, the two sets of signals are added together to realize UNRZ data. An advantage of the present invention is that PDHB, PDL PMD and PGD are eliminated. PMD is eliminated by adjusting polarization mode delay at the transmitter, fiber, receiver or any one or more or all of the above. PDHB is reduced as |Eox and Eoy| will have equal average power at the receiver as the net vector sum constitutes UNRZ. Also, because of the transmission of constant envelopes XPM, SPM and FWM are reduced. A further advantage of the present invention is that the overall bandwidth of the transmitted data is the same as UNRZ.